Emergy analysis of the blue and green water resources in crop production systems

Water consumption in grain production consists of blue (i.e., surface water and groundwater) and green water (i. e., rainwater), however, is rarely distinguished in existing emergy analysis. Taking four major grain crop production systems (rice, wheat, maize, and soybean) by provinces in mainland China over the period 2001-2016 as the study case, in this study the blue and green water were differentiated, for the first time, in the emergy analysis for crop production systems based on water footprint assessment. Three indicators were estimated per crop per province per year: water resource emergy input (WEI), water resource emergy contribution ratio (WECR), and water resource emergy use efficiency (WEUE). Results show that during the study period, the total WEI into the major grain crop production systems increased by 16.61%; blue and green WEI accounted for 52.64% and 47.36%, respectively, by 2016. Although the blue WECR was higher than green WECR, it decreased significantly over the study period. The annual average green WEUE was 10.99 sej sej-1, 7.7 times the blue; moreover, it increased by 27.28% over the study period. The annual average total WECRs in nine provinces were higher than the national average, but the corresponding total WEUEs were lower than the national average; of these, four were distributed in arid and rainwater-deficient areas of northwest China. The emergy analysis of blue and green water separately provides information for a more precise and efficient water allocation as well as better cropping structures given corresponding water endowments.